This invention relates to a method of press-forming a planer sheet of corrugated paperboard produced by utilizing a thermoplastic adhesive into an at least partly curved board which serves as the substrate of a wall covering material for the interior of cars, ships or buildings and an apparatus for performing the method.
As disclosed in U.S. Pat. No. 4,093,482, corrugated paperboard is useful as a basic material for wall covering board materials to afford comforts and/or decorative effects to the interior of cars, ships or buildings because of its light weight and good heat and sound insulating ability. This fact has attracted increasing attention in related industries. In automobiles, for example, trim boards such as roof trims and door trims of corrugated paperboard base have already been in practical use on industrial scale.
Trim boards of this type usually consist of a corrugated paperboard substrate and a skin or facing layer which is laid on one side of the substrate for producing protective, decorative and/or cushioning effects. In most cases double-faced corrugated paper board is used as the material of the substrate, while a variety of soft and pliable sheet materials are useful as the facing layer: examples are plastics sheets, artificial leathers, woven or nonwoven cloths and laminated materials given by backing any of these sheet materials with a cushioning layer such as a urethane foam layer.
When a wall or panel to be covered with a trim board of the above described type has a curved surface as is usual in automobile roofs, it becomes a principal step in the production of the trim board to shape a planer sheet of corrugated paperboard into an at least partly curved board, which serves as the substrate of the trim board, in compliance with the curved surface of the object. Such shaping of corrugated paperboard is accomplished by application of pressure through male and female dies appropriately arranged in a press, and then the facing layer is bonded to the shaped substrate.
Considering that corrugated paperboard is inherently poor in ductility, the above quoted U.S. Pat. No. 4,093,482 requires that corrugated paperboard as the substrate material be of the type comprising a thermoplastic resin as the adhesive to bond paper liners to a corrugated paper medium and that press-forming of the corrugated paperboard substrate be performed with application of heat to the material, i.e. by means of a hot-press. According to this patent, the thermoplastic resin adhesive in the corrugated paperboard subjected to press-forming is sufficiently softened or fluidized and, as a consequence, permits a certain extent of relative displacement of the corrugated medium and the individual liners while pressure is applied thereto, so that the corrugated paperboard can be shaped smoothly without leaving unwanted strains therein. Since the thermoplastic resin solidifies after completion of the press-forming, the curved substrate can retain its shape and resist deformation by external forces and changes in the environmental condition. Thus, it is an ingenious point of this method that the thermoplastic resin serves as a sort of lubricant during press-forming but affords rigidity to the shaped substrate.
Sometimes use is made of a special type of corrugated paperboard whose paper liners (optionally corrugated medium, too) are impregnated with a thermoplastic resin for the enhancement of stability and moisture resistance as the substrate material. This type of corrugated paperboard too can be press-formed by the method according to U.S. Pat. No. 4,093,482 since the application of heat to the corrugated paperboard during press-forming causes not only fluidization of the thermoplastic adhesive but also softening, i.e. lowering of rigidity, of the resin-impregnated liners.
However, a curved substrate of corrugated paperboard produced by the above described method is often unsatisfactory in its appearance because wrinkles tend to appear in curved regions particularly on one side which gives a concave surface. The probability of such wrinkling increases when the corrugated paperboard is subjected to relatively deep drawing or severe bending with a comparatively small bend radius. According to my recognition, a primary reason for such wrinkling is that actual bending of the substrate material begins while heating of the substrate material is yet insufficient.
In a hot-press for the press-forming operation, both male and female dies are heated so that heating of the substrate material is effected by heat conduction through these dies. Usually the female die is kept stationary. When the board is placed on the stationary female die a large portion of the board is spaced from the shaped face of this die. Then the male die is lowered so as to press the board against the female die, but during an initial stage of pressing the contact of the shaped face of the male die with the planer board is established only in a limited area, so that the board begins to undergo bending even in regions still spaced from the male die, i.e. in regions not yet heated by the male die. Accordingly it is highly probable that the corrugated paperboard undergoes bending in some regions before fluidization of the thermoplastic adhesive therein with the result that wrinkles appear in concaved regions of the liner contacted with the male die. Even when the wrinkled liner is laid with a facing layer, the outer surface of the facing layer tends to show visible traces of the wrinkles since the facing layer is a thin sheet of a soft and pliable material. The presence of such traces of course impairs the decorative effect and hence commercial value of the trim board.